The New Zealand glow-worm is one of the most interesting insects of the New Zealand fauna. It occurs throughout the country in limestone caves, unused mining tunnels, along stream banks, in damp bush-clad ravines, in damp shady crevices, and under tree-fern fronds in rain forests. The Glow-worm Grotto in Waitomo Cave has become world famous because of the tens of thousands of glow-worm larvae which live on the walls, ceilings, and stalactites of the grotto.
The insect is not related to the European glowworm which is a beetle. The New Zealand glowworm is a fly belonging to the gnat family. The larvae, pupae, and adults of both sexes are all luminous. In the larval stage the light attracts prey in the form of other organisms, while in the pupal and adult stages the light attracts the opposite sex.
The larva prepares a nest in the form of a tunnel of mucous and silk, and suspends from this an array of fishing lines composed of the same materials. Prey is snared in the long sticky fishing lines. The larva hauls up the fishing line on which the prey is entangled and consumes the trapped insect. Up to 70 lines are let down by one larva and, depending on the size of the larva, the lines vary in length from under 1 cm to 50 cm. Each fishing line consists of a long thread of silk which bears at regular intervals a series of mucous droplets giving the appearance of a string of beads. The droplet size is about 1 mm in diameter. Nests and lines can be reconstructed and repaired. Fully grown larvae measure up to 40 mm in length and adult flies average 15 mm in length. The life cycle appears to take 11–12 months, with the larval stage lasting eight or nine months. Breeding shows little evidence of being a seasonal phenomenon.
by Roy Alexander Harrison, D.SC., Senior Lecturer in Agricultural Zoology, Lincoln Agricultural College.